A very important subject. Too bad Linus Pauling forgot about it, as that cost him a third Nobel prize. (The base structures in DNA undergo a keto-enol rearrangement and he had the wrong form in his structure.)

And this also is critical to understanding the misnomenclature behind the polymer poly(vinyl alcohol). Vinyl alcohol doesn’t exist except as acetaldehyde, so poly(vinyl alcohol) is made by hydrolysis of poly(vinyl acetate).

Why is the keto form generally favored over enol form? I mean, you’ve mentioned how the enol form is stabilized (conjugation, aromaticity, and H-bonding), but why is the enol form less stable to begin with?

The keto form is favored under neutral conditions. This is because RO-H is more acidic than RC-H (oxygen is more electronegative) and thus the keto form (which has the acidic hydrogen on the alpha carbon rather than the carbonyl oxygen) is favored.

Conversion of the keto to the enol form is catalyzed by acidic/basic conditions. Acidic conditions will protonate the carbonyl oxygen while basic conditions will deprotonate the alpha carbon, both of which generates a charged intermediate which is stabilized by conversion to the enol form.

True. If you have a carbonyl two carbons away, the enol form can be stabilized by intramolecular hydrogen bonding. The enol form can also be stabilized if it happens to be aromatic (as in phenol). Finally, subtle factors like solvent, increasing the number of alkyl substituents or adding electron withdrawing groups can also affect the keto/enol ratio.

About Master Organic Chemistry

After doing a PhD in organic synthesis at McGill and a postdoc at MIT, I applied for faculty positions at universities and it didn’t work out, yada yada yada. So I decided to teach organic chemistry anyway! Master Organic Chemistry is the resource I wish I had when I was learning the subject.